High-tension cable insulated with synthetic tape and oil



De 14, 1965 P. G. PRIAROGGIA HIGH-TENSION CABLE INSULATED WITH SYNTHETICTAPE AND OIL Filed Feb. 27, 1962 INVENTOR. PAOLO Gazz/:INA PQ/AEOG G/A rwwmwmhmN United States Patent O HIGH-TENSION CABLE INSULATED WITHSYNTHETIC TAPE AND OIL Paolo Gazzana Priarogga, Milan, Italy, assignorto Pirelli Societa per Azioni, Milan, Italy, a corporation of ItalyFiled Feb. 27, 1962, Ser. No. 175,991

Claims priority, application Italy, Mar. 31, 1961,

5,950/ 61 8 Claims. (Cl. 174-25) The present invention relates to ahigh-tension cable for direct or alternating current in which theinsulation is partly constituted by synthetic plastic material.

In cables of this kind the insulation is generally constituted byhelically Wound thin tapes of plastic material and by mineral oilfilling the spaces existing between said tapes. As the syntheticmaterial is not permeable to the oil, it is very difficult to carry outa good impregnation, so that air bubbles can easily be retained betweenthe tapes.

As is known, the presence of air in the cable insulation is veryharmful, as it can easily ,give rise to the known ionization phenomenawhich seriously impair the cable life and do not allow the retention ofits ability to withstand high tensions. In order to solve the problem ofa good oil circulation, the British Patent No. 854,797 provides forinstance, in the preparation of the insulation, the use of tapes ofsynthetic plastic material previously roughened on at least one of theirsurfaces by means of a suitable treatment. This treatment consists inObtaining on said surfaces parallel and straight grooves, disposedtransversally with respect to the edges of the tape and extending forits whole width. These grooves are provided in great number and have adepth ranging between 1 micron and 10 microns. In this way it ispossible to have a good mobility of the oil between thei tapes, as iteasily penetrates between the facing surfaces, and the presence of airis eliminated. However, this expedient has some disadvantages which makeits application unsuitable and which are in practice due to the methodaccording to which the winding tapes are prepared.

The roughened tapes are in fact obtained by passing a sheet of syntheticmaterial between two rollers, which may be heated, one of which isprovided along its periphery with suitable projections which, pressingon the sheet, engrave the grooves, and the sheet is then cut into tapes.This particular method requires the adoption of a special machinery andaffects in a considerable way the cost of the tapes. Furthermore, it isto be noted that the sur-face of the tape, owing to the presence of thegrooves, has sharp corner-edges, two for each groove.

At the corner-edges, there are high voltage gradient concentrations, andthis does not allow the employment of very high service gradients, ifthe formation of undesired concentrations are to be avoided.

The object of the present invention is to provide a high-tensionelectric cable for direct or alternating current in which the dielectricis constituted by insulating tapes of synthetic plastic material incon-tact with a film of impregnating oil, and in which the aboveindicated drawbacks are eliminated.

The cable of Ithe present invention is characterized in that thinseparating and insulatin-g layers allowing a very good penetration ofthe impregnating oil are interposed between the insulating tapes. Saidseparating layers can be constituted by a winding of individual threads,of thin fabric tapes or by a continuous braid of threads of syntheticplastic material resistance to the impregnating oil of the cable.

Further details of the invention will more clearly appear from thefollowing description and from the at- 3,223,773 Patented Dec. 14, 1965ICC tached drawing, which shows examples of the invention and in which:

FIG. 1 is a perspective view, with certain parts removed, of a cablecore constructed in accordance with the invention; and

FIG. 2 is a perspective View, with certain parts removed, of a furtherembodiment of cable core constructed in accordance with the invention.

In FIGURE l, the reference numeral 1 indicates the stranded conductoronto which there is wound a screening tape 2 of a known type, such as aconductive tape. Around the screening tape 2 there is a first winding 3constituted by individual threads of synthetic material. This firstwinding 3 and the remaining windings 3 constitute thin separatinginsulating layers, composed of a plurality of individual threads,helically wound at a suitable pitch about the underlying tapes. Aboutthe first separating layer 3, covering the screening tape 2, there isthen wound an impervious insulating tape 4, and the building up of thecable is carried on by alternating separating layers 3 and insulatingtapes 4 until the desired thickness of the dielectric is obtained. Theempty spaces existing between the threads constituting the separatinglayers and those existing between the edges of the tapes 4 are in thisway easily filled by the insulating oil which is intended to impregnatethe cable.

Instead of resorting to a winding of threads of synthetic plasticmaterial to form the separating layers 3, each individual tape 2 or 4can be covered with a woven fabric tape constituted by threads ofsynthetic material, or else each individual tape can be covered with acontinuous braid, also of synthetic material, which can be applied withknown methods.

FIG. 2 represents a cable core obtained by using a -fabric for thepreparation of the separating layers. In said figure 1' indicates thestranded conductor, 2 the screening tape, 3' the separating layerscomposed of a thin fabric of synthetic material and 4 the impreviousinsulating tapes. Also, in this case, a very good penetration of theimpregnating oil between the various layers 4 constituting the soliddielectric is obtained.

The plastic material constituting the separating layers 3 or 3 is asynthetic plastic material and in a preferred form of the invention,said material has the same characteristics as those of the insulatingtapes 4 and 4' of FIGS. 1 and 2.

Since, as is known, an oil film has a dielectric strength which is thehigher the smaller is its thickness, the necessity arises of adoptingvery reduced diameters and thicknesses respectively for the threadsforming the separating layers and for the fabric or braid, in order tominimize the thickness of the oil gap between the tapes 4 and tomaintain therefore a high dielectric strength of the oil films existingin the cable insulation.

As a preferred value, a diameter of 0.025 mm. for the individual threadshas been found suitable. Therefore, when a fabric is used in lieu ofthreads to form the separating layers 3', the maximum thickness of saidfabric is 0.05 rnrn.

When the separating layers 3 are formed with tapes obtained from afabric, it has been found convenient to use tapes having a width equalto that of the insulating tapes 4 as shown in FIG. 2 and to overlap themexactly, during the wrapping operation, on the underlying insulatingtapes 4.

When the separating layers are formed with individual threads, a certainnumber of sufficiently close threads is wrapped on the tapes 4 at asuitable pitch in order to cover the underlying tape for its entirewidth.

The materials to be used for preparing the insulating tapes 4 or 4 andthe separating layers 3 or 3 are necessarily those resistant to theaction of the oil existing in the cable, and may, for example, bepolypropylene, polytetrafluoroethylene, polyethylene terephthalate,polybutylene, polystyrene and polycarbonates. These materials, besidesbeing resistant to the action of the oil at room temperature, are notaffected by it even at the high service ternperatures of these cables,which can exceed 80 C. They have, moreover, a high dielectric strength.

Furthermore, polypropylene, polytetraffuoroethylene and thepolycarbonates, besides having low dielectric losses, possess a reduceddielectric constant, slightly different from that of the oil andconsequently they are particularly suitable for the alternating currentcables.

The cables obtained as described above have the advantage of eliminatingthe inconveniences encountered in conventional cables of the same type.In fact, by means of a suitable selection of the fabric thickness or ofthe diameter of the individual threads, an oil layer of well definedthickness can be interposed between the insulating tapes 4 or 4' inorder to comply with the two requirements of a high value of thedielectric strength and of a sufficient mobility of the oil, whichrequirements are in conflict with each other.

The individual threads and the threads forming the fabric or braid havesurfaces of curvature considerably greater than those existing at thecorner-edges of the grooves indicated in the above mentioned BritishPatent, and this contributes to proportionally reducing theconcentrations of electric stresses, thus allowing the adoption ofhigher working voltages,

Obviously, the cables prepared in accordance with the present inventioncan be used either for alternating current or for direct current.

It is understood that the above described embodiments of the inventionare given only by way of non-limiting examples and that the inventionincludes other forms of cables based on the foregoing principles andincluded within the scope of the appended claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A high-tension electric cable comprising a conductor surrounded bylayers of fluid impermeable insulating tape of synthetic plasticmaterial, fabric tape disposed intermediate each of said layers and thenext layer and separating each of said layers from the others throughouttheir lengths, said fabric tape having the same width as the insulatingtape and being wrapped around the underlying insulating tape inoverlapping relation and with the edges of adjacent turns of the fabrictape abutting each other and said fabric tape being formed of individualfilaments of synthetic plastic material which are thin relative to thethickness of said insulating tape, said filaments being disposed in apredetermined ordered arrangement throughout the length of said cablewith at least portions of their periphery separated from each other andthe adjacent insulating tape to provide fluid passageways therebetweenand between the adjacent layers, each of which passageways extends fromone end to the other end of said cable, and an insulating fluid fillingsaid passageways, said synthetic plastic material being resistant to theaction of said fluid.

2. A high-tension electric cable comprising a conductor surrounded bylayers of fluid impermeable insulating tape of synthetic plasticmaterial, fluid impermeable individual filaments of synthetic plasticmaterial which are thin relative to the thickness of said tape disposedintermediate each of said layers and the next layer and separating eachof said layers from the others throughout their lengths, said filamentsbeing braided in the form of a continuous textile braid and beingdisposed in a predetermined ordered arrangement throughout the length ofsaid cable with at least portions of their periphery separated from eachother and the adjacent tape to ,provide fluid passageways therebetweenand between the adjacent layers, each of which passageways extends fromone end to the other end of said cable, and an insulating fluid fillingsaid passageways, said synthetic plastic material being resistant to theaction of said fluid.

3. A high-tension electric cable comprising a conductor surrounded by aplurality of layers of oil impermeable insulating tape of syntheticplastic material, each of said layers being formed by a single thicknessof said tape, at least one oil impermeable, continuous filament ofsynthetic plastic material which is thin relative to the thickness ofsaid tape disposed intermediate and in contact with each of said layersand the next layer and separating each of said layers from the othersthroughout their lengths, said filament being disposed in apredetermined ordered arrangement throughout the length of said cablewith at least portions of its periphery separated from other portionsthereof and from the adjacent tape to provide lengthwise andcircumferential fluid passageways between the adjacent layers, each ofwhich passageways extends from one end to the other end of said cable,and an insulating oil filling said passageways.

4. A high-tension cable as set forth in claim 3, wherein a single lsaidfilament is intermediate said layers and is helically wound around theinner layer with the turns thereof abutting.

5. A high-tension cable as set forth in claim 3, wherein said filamenthas a diameter substantially no greater than 0.025 millimeter and thematerial thereof is selected from the group consisting of polypropylene,polytetrafluoroethylene, polyethylene terephthalate, polybutylene,polystyrene and polycarbonates.

6. A high-tension cable as set forth in claim 3, wherein a plurality ofsaid filaments are interwoven to form a fabric disposed intermediatesaid layers.

7. A high-tension cable as set forth in claim 3, wherein a plurality ofsaid filaments are braided together and are disposed intermediate saidlayers.

8. A high-tension electric cable comprising a conductor surrounded by aplurality of layers of fluid impermeable insulating tape of syntheticplastic material, each of said layers being formed by a single thicknessof said tape, at least one fluid impermeable, continuous filament ofsynthetic plastic material which is thin relative to the thickness ofsaid tape disposed intermediate and in contact with each of said layersand the next layer and separating each of said layers from the othersthroughout their lengths, said filament being disposed in apredetermined ordered arrangement throughout the length of said cablewith at least portions of its periphery separated from other portionsthereof and from the adjacent tape to provide lengthwise andcircumferential fluid passageways between the adjacent layers, each ofwhich passageways extends from one end to the other end of said cable,and an insulating fluid filling said passageways.

References Cited by the Examiner UNITED STATES PATENTS 451,586 5/1891Williams 174-24 524,066 8/1894 Macfarland 174-120 1,574,076 2/1926Fisher 156-48 e3 ,077,510 2/1963 Olds 174-25 3,077,514 2/1963 Kang174-25 X FOREIGN PATENTS 147,915 9/ 1926 Switzerland.

OTHER REFERENCES Uhle: German printed application No. 1,020,075 (1 sht.dwg., 3 pp. spec), 11/1957.

JOHN F. BURNS, Primary Examiner.

JOHN P. WILDMAN, E. JAMES SAX, LARRAME A.

ASKlN, Examiners.

3. A HIGH-TENSION ELECTRIC CABLE COMPRISING A CONDUCTOR SURROUNDED BY APLURALITY OF LAYERS OF OIL IMPERMEANBLE INSULATING TAPE OF SYNTHETICPLASTIC MATERIAL, EACH OF SAID LAYERS BEING FORMED BY A SINGLE THICKNESSOF SAID TAPE, AT LEAST ONE OIL IMPERMEABLE, CONTINUOUS FILAMENT OFSYNTHETIC PLASTIC MATERIAL WHICH IS THIN RELATIVE TO THE THICKNESS OFSAID TAPE DISPOSED INTERMEDIATE AND IN CONTACT WITH EACH OF SAID LAYERSAND THE NEXT LAYER AND SEPARATING EACH OF SAID LAYERS FROM THE OTHERSTHROUGHOUT THEIR LENGTHS, SAID FILAMENT BEING DISPOSED IN APREDETERMINED ORDERED ARRANGEMENT THROUGHTOUT THE LENGTH OF SAID CABLEWITH AT LEAST PORTIONS OF ITS PERIPHERY SEPARATED FROM OTHER PORTIONSTHEREOF AND FROM THE ADJACENT TAPE TO PROVIDE LENGTHWISE ANDCIRCUMFERENTIAL FLUID PASSAGEWAYS BETWEEN THE ADJACENT LAYERS, EACH OFWHICH PASSAGEWAYS EXTENDS FROM ONE END TO THE OTHER END OF SAID CABLE,AND AN INSULATING OIL FILLING SAID PASSAGEWAYS.